1. Field of the Invention
The present invention is in the field of optical waveguide components, such as optical fibers, and, in particular, is in the field of fiber gyroscopes, rotation sensors and interferometers using superfluorescent fiber laser sources and the like.
2. Description of the Related Art
A Sagnac interferometer comprises an optical loop, typically of optical fiber that is used to sense rotation of an object onto which the loop is mounted. Such interferometers operate by dividing the optical energy from a light source into two substantially equal beams of light and causing the two beams of light to propagate around the loop in opposite directions. The two beams of light are combined after passing through the loop and are detected by a detector after passing through a directional coupler. The changes in intensity of the combined light caused by interference of the two beams are detected. In accordance with the well-known Sagnac effect, rotation of the object and thus of the loop of fiber causes changes in the relative phase between light propagating in the two directions which in turn causes the detected intensity to change. The rotation rate of the loop can be determined from the detected changes in the intensity. See, for example, U.S. Pat. Nos. 4,410,275; 4,529,312; 4,637,722; 4,687,330 and 4,836,676.
The light source of the interferometer is preferably a broadband source. Resonant fiber lasers (RFL's) and superfluorescent fiber lasers (SFL's) are capable of wide spectral bandwidth and high power output; such devices have better average wavelength stability versus temperature than for semiconductor diode sources. These properties, together with prospects for long life, have made them alternatives to standard superluminescent diode sources. Resonant fiber lasers offer the highest ratio of output power to pump power. Double pass superfluorescent lasers offer intermediate values and single pass superfluorescent lasers give the lowest values. All of those sources have shown susceptibility to optical feedback, resulting in large gyro instability and output errors with the resonant laser fiber source, onset of instability at low source output levels for the double pass superfluorescent fiber lasers, and similar behavior at higher but still less than optimum source output for the single pass superfluorescent fiber laser.
For example, the gyroscope disclosed in U.S. Pat. No. 4,637,025 uses a broadband light source to provide the light introduced into the loop of optical fiber. The light source in U.S. Pat. No. 4,637,025 operates by introducing a pump signal into a single-mode optical fiber having a core doped with an active fluorescent material such as neodymium or other rare earths. The pump light has a sufficient intensity to cause amplification of spontaneous emission of photons by the fluorescent material. In one embodiment (FIG. 1), pump light is input into the optical fiber via a lens. In the second of the two embodiments, the pump light is introduced via a dichroic lens that is transparent to the pump light and highly reflective of emitted light. The pump light is absorbed by the fluorescent material and excites the electrons therein to higher energy states resulting in the emission light when the electrons transition to lower states. Because of the random manner in which the spontaneous emissions occur, the amplified emitted light is effectively spontaneous fluorescence and temporally incoherent.
In order to reduce the absorption losses caused by intermediary optical devices, in particular of couplers, proposals have been made to omit the directional coupler isolating the light source from the detector. In U.S. Pat. No. 4,842,409, the light source and the photodetector are arranged collinearly, either in the form of a single semiconductor diode which is used alternatively as an emitter and as a detector of light energy, or else, by being aligned. In the latter arrangement, the light source being constituted by a semiconductor diode is coupled via both its front face and its rear face and is interposed between the photodetector and the Y-coupler (integrated optic devices are used in that patent rather than conventional optical devices). The diode is therefore used alternatively as an emitter and as an amplifier of light energy. In either case, the semiconductor diode is switched so as periodically to emit light pulses which are as long as possible, i.e. of duration just less than the time .tau.' required for the two beams to propagate over their entire go-and-return paths. The switching period 2.tau.' is then very close to the period 2, of the phase modulation used for optimizing detection sensitivity since the go-and-return path length covered by the beams between the light source and the interferometer loop is small compared with the path link of the interferometer loop itself. As a result, in the output signal from the photodetector, the various components due to the modulation coming from source switching and coming from phase modulation overlap one another, thereby making it difficult to detect the useful signal. In the somehow averted by artificially doubling the propagation time of the two beams on their go-and-return paths by adding an additional length of optical fiber between the source and the Y-coupler, the additional length being equal to one fourth of the length of the interferometer loop. However, adding a significant length of optical fiber increases the bulk of the interferometer system and reduces light energy efficiency.
In U.S. Pat. No. 4,848,910, another solution is proposed to the above problem. That solution is essentially of an electronic nature and does not use the optical properties of the components. In order to obtain good energy efficiency, according to that reference, it is necessary to use emit and receive light pulses of maximum duration .tau.' corresponding to twice the transit time .tau.'' taken by the light to go from the laser diode to the interferometer ring plus the transit time .tau. around the interferometer ring. As a result, the emit-receive switching period 2.tau.' is very close to the phase modulation period 2.tau.. The spectrum of the signal generated by the photodetector includes spectrum lines in the vicinity of the frequency 1/2.tau. caused by the modulation due to emit-receive switching of the laser diode. Those spectrum lines disturb the detection of the useful spectrum line at 1/2.tau. generated by the phase modulation. U.S. Pat. No. 4,848,910 proposes to solve that problem by isolating the useful signal in a different spectrum line of the signal coming from the photodetector after the phase modulation has been combined with the modulation resulting from laser diode emit-receive switching or more generally with any amplitude modulation of the light energy emitted by the laser diode. The optical power coming from the interferometer ring has a frequency spectrum which is very rich in harmonics. So long as the two modulation frequencies are slightly different, there exists a component that may be used for measuring the relative phase difference of the two beams with maximum sensitivity for small phase differences.
Unlike the two solutions proposed in the two abovementioned U.S patents, the present invention solves the above-mentioned problem without the need for switching the light source. The object of the present invention is also to solve the above-mentioned problem by using interesting optical properties of superfluorescent laser sources used as signal amplifiers.